Recent advances have been made in casting of materials, such as silicon, for applications in the photovoltaic industry. Such advances are described, for example, in copending application Ser. Nos. 11/624,365 and 11/624,411, filed Jan. 18, 2007. Materials, such as those used to form semiconducting substrates or wafers, may include combinations of elements from Groups II-VI, III-V, and IV-IV. In addition, cast metals and especially those reactive metals melted in vacuum may be included as materials. As used herein, the term “material,” unless otherwise specified, includes any element or combination of elements from Groups II-VI, III-V, and IV-IV, or those elements from the alkali, alkaline or transition metals and in particular those which may be formed into semiconductor wafers or substrates.
During casting processes, for example, the material may exist simultaneously in multiple phases, such as a molten or partially melted material containing a liquid portion and a solid portion. A solid-liquid interface is located between the liquid and solid portions until the material is completely solidified. As used herein, the term “solid-liquid interface” refers to the boundary between the liquid and solid portions of a material, for example, during either the melting or solidification portions of a casting process. It is understood that the solid-liquid interface may not be exactly two-dimensional, and may have a finite thickness depending on the material being melted/solidified and other processing conditions. Furthermore, the interface may be flat or have a curved shape. Monitoring the solid-liquid interface is important to controlling the melting and solidification processes during casting, so that certain crystal growth characteristics may be achieved, for example. In another example, monitoring the depth of a liquid being held in a container, such as a crucible or holding tank, is important where the height of the column of liquid cannot be determined by only knowing the location of the free liquid surface.
In a known casting procedure for the manufacture of photovoltaic cells, a material, such as silicon feedstock, may be mixed with a dopant for inducing either a positive or negative conductivity type, melted, and then crystallized by either pulling the crystallized material out of a melt zone or solidifying it in place to form ingots. If silicon feedstock is used, these ingots may be monocrystalline silicon (via the Czochralski (CZ) or float zone (FZ) methods), or cast into blocks or “bricks” of monocrystalline silicon, multi-crystalline silicon or polycrystalline silicon, depending on the grain size of the individual silicon grains. As used herein, the term “cast” means that the silicon is formed by cooling a molten material in a mold or vessel used to hold the molten material. As used herein, the term “monocrystalline silicon” refers to a body of single crystal silicon, having one consistent crystal orientation throughout. Further, “conventional multi-crystalline silicon” refers to crystalline silicon having cm-scale grain size distribution, with multiple randomly oriented crystals located within a body of silicon. As used herein, however, the term “geometrically ordered multi-crystalline silicon” (hereinafter abbreviated as “geometric multi-crystalline silicon”) refers to crystalline silicon, having cm-scale grain size distribution of geometrically shaped crystals, with multiple ordered crystals located within a body of silicon. Further, as used herein, the term “poly-crystalline silicon” refers to crystalline silicon with micron order grain size and multiple grain orientations located within a given body of silicon. For example, the grains are typically an average of about submicron to submillimeter in size (e.g., individual grains may not be visible to the naked eye), and grain orientation distributed randomly throughout. In the casting procedure described above, the ingots or blocks are cut first into bricks with the proper cross-section, and then into thin substrates, also referred to as wafers, by known slicing or sawing methods. These wafers may then be processed into photovoltaic cells.
Conventional monocrystalline silicon for use in the manufacture of photovoltaic cells is generally produced by the CZ or FZ methods, both being processes in which a cylindrically shaped boule of crystalline silicon is produced. For a CZ process, the boule is slowly pulled out of a pool of molten silicon. For a FZ process, solid material is fed through a melting zone and re-solidified on the other side of the melting zone. A boule of monocrystalline silicon, manufactured in these ways, contains a radial distribution of impurities and defects, such as rings of oxygen-induced stacking faults (OSF) and “swirl” defects of interstitial or vacancy clusters. These defects are fairly well understood, and monocrystalline silicon is generally a preferred source of silicon for producing photovoltaic cells, because it can be used to produce high efficiency solar cells. Monocrystalline silicon is, however, more expensive to produce than conventional multi-crystalline silicon, using known techniques such as those described above.
Conventional multi-crystalline silicon for use in the manufacture of photovoltaic cells is generally produced by a casting process. Casting processes for preparing conventional multi-crystalline silicon are known in the art of photovoltaic technology. Briefly, in such processes, molten silicon is contained in a crucible, such as a fused silica or quartz crucible, and is cooled in a controlled manner to permit the crystallization of the silicon contained therein. The block of multi-crystalline silicon that results is generally cut into bricks having a cross-section that is the same as or close to the size of the wafer to be used for manufacturing a photovoltaic cell, and the bricks are sawn or otherwise cut into such wafers. The multi-crystalline silicon produced in such a manner is an agglomeration of crystal grains where, within the wafers made therefrom, the orientation of the grains relative to one another is nearly random, although some orientations are preferred. Photovoltaic cells made from multi-crystalline silicon generally have lower efficiency compared to equivalent photovoltaic cells made from monocrystalline silicon, due to a higher concentration of grain boundary and dislocation defects. However, because of the relative simplicity and lower costs for manufacturing conventional multi-crystalline silicon, as well as effective defect passivation in cell processing, multi-crystalline silicon is a more widely used form of silicon for manufacturing photovoltaic cells.
Recently, high quality geometrically ordered multi-crystalline silicon has been produced by a casting process, yielding large volumes of cast geometrically ordered multi-crystalline silicon that does not have a random distribution of grains therein. Additionally, high quality monocrystalline silicon has also been produced by a casting process, yielding large volumes of cast monocrystalline silicon that is free of both the high levels of dislocations and grain boundaries found in multicrystalline cast silicon and the radial distribution of defects and impurities present in the CZ and FZ methods. See, for example, copending U.S. patent application Ser. Nos. 11/624,365 and 11/624,411.